The present invention relates to a process for electro-dipcoating that is particularly suitable for producing coating films having a desired degree of matteing and improved edge protection, as well as the electrodeposition lacquers suitable therefor. Electro-dipcoating is a fully automated, environmentally friendly and economic application method and is therefore used in practice in the mass production lacquering of electrically conducting surfaces, in particular metal surfaces. Electro-dipcoating is a fully automated application method with a high application efficiency. The process preferably takes place in closed cycles and permits the recycling of excess lacquer material as well as the auxiliary substances and process materials that are used.
In order to improve the corrosion resistance of the coating films, corrosion-inhibiting pigments such as lead or chromium compounds, for example lead chromate, basic lead silicate or strontium chromate, are often added to the electrodeposition lacquers. These compounds are however not desirable for health and environmental safety reasons.
The pigments such as zinc phosphates, aluminium phosphates, zinc oxide, etc. that have been proposed as being less toxic do not provide satisfactory corrosion prevention results. Also, the zinc compounds in the electro-dipcoating baths may lead to instabilities.
The addition of lanthanum salts of organic acids such as for example lanthanum acetate is accordingly proposed in DE-A-43 06 024. However, no satisfactory results can be obtained as regards edge protection.
Crosslinked polymer microparticles based on epoxy resins are described in DE-A-42 25 999, which are used as organic fillers, matteing agents and/or reinforcing agents in conventional electrodeposition lacquers for priming vehicle bodies. These electrodeposition lacquers do not provide satisfactory results as regards edge protection either.
EP-A-0 240 876, EP-A-0 216 337, WO 93/07206 and EP-A-0 174 628 describe electro-dipcoating with baths to which phosphoric acid epoxy esters are added. The latter are produced from epoxy compounds with phosphoric acid in the presence of water. It has been found that these baths have an insufficient bath stability and that a relatively high redissolution of the deposited lacquer material takes place in the rinsing zones of the paint shops.
The object of the present invention was accordingly to provide a process for anodic electro-dipcoating that does not have these disadvantages and that produces lacquer films having improved properties compared to the prior art, in particular as regards an improvement in the edge protection, and moreover permits an arbitrary adjustment of the degree of matteing and is in addition highly versatile, in which the baths are also stable and the redissolution is reduced.